A need for a stable reference voltage is common in the design of electronic equipment. Nearly all electronic circuits require one or more sources of stable DC voltage. Bandgap voltage reference circuits are commonly used to provide a stable DC reference voltage.
A bandgap voltage reference circuit generally employs two transistors operated at different current densities. Typically, the bases of the two transistors are tied together and a resistor connects their emitters, to sense the difference in base-emitter voltages between the two transistors.
Also, the base-emitter voltage of a transistor exhibits a temperature-dependent function. A bandgap circuit typically generates a voltage with a positive first-order temperature coefficient that is approximately the same as the negative first-order temperature coefficient of the base-emitter voltage. However, the bandgap voltage may still have a temperature dependency for temperature coefficients higher than the first order. The second-order non-linearity of a bandgap voltage reference circuit is generally referred to as “curvature”.
Some applications require a stable and accurate reference voltage over a large range of temperatures. In the past, acquiring such accuracy typically involved testing and trimming of an integrated circuit after it had been fabricated and assembled. Alternatively, testing and trimming can occur before assembly, or before and after assembly.